An important endodontic procedure, known as a “root canal” procedure, involves removing organic material from the root canals of an infected tooth and filling the canal with an inert obturating material such as gutta percha gum.
An effective root canal procedure avoids extraction of the infected tooth. In this procedure, a dentist or endodontist utilizes a series of endodontic instruments, for example files, for the debridement, cleaning and sterilization of the root canal. These files are rotated within the canal to clean the canal surfaces, removing debridement (organic) material in the process, facilitating improved irrigation, and in some cases shaping the canal for easier filling with the obturating material.
Root canal preparation, and root canal retreatment (to repair a defective root canal procedure), are typically effected by motor-driven instruments such as files. Root canal retreatment can be defined as a procedure to remove root canal filling materials from the tooth, followed by cleaning, shaping and obturating the canals.
Files used for debridement and removal of organic material, which are usually made of stainless steel or nickel titanium, or from modifications of stainless steel or nickel titanium, or from any other material or combination of materials which is generally rigid and allows the file to progress along the canal. Such files work like augers to move material out of the root canal via a helical groove. This effectively makes the file behave like a screw, driving forward when rotated in the forward direction (which may for example, depending upon the orientation of the threads, be the counter-clockwise direction) and backing off when rotated in the reverse (for example clockwise) direction. However, the threads defining the helical groove can lock or catch on interior canal surfaces, especially in constricted and/or curved parts of the canal. If too much force is applied to the file at such points the file can break, necessitating removal of the broken piece of file which can be a difficult procedure which could ultimately result in extraction of the tooth, effectively obviating the benefit of the root canal procedure,
Motor-driven nickel-titanium files are widely used in a continuous rotation mode for the canal preparation. They offer significant advantages compared to hand-held instruments: they are faster, they make the procedure easier and therefore require a shorter learning curve, and they better maintain the canal curvature. However, instrument fracture, which can adversely affect the outcome of a root canal treatment, is a concern. When used in a continuous rotation mode, the instrument can bind in the canal. It will then be subjected to torsional stress, mainly at its tip. The dentist is typically not aware that the instrument is binding. The motor keeps rotating the instrument, and the torsional stress on the instrument will increase with the degree of rotation until a level high enough to fracture the instrument is reached. Instrument fracture will then occur. This is known as ‘torsional fracture’ or fracture from binding. Even without binding, the repeated torsional stress to which the instrument is subjected while engaging the canal walls and cutting tooth structure will in time cause fatigue of the instrument material, and instrument fracture from torsional fatigue will eventually occur.
The degree of rotation at which the instrument fractures is called the angle at fracture. The angle at fracture can be determined for any instrument, and at any part of the instrument. Usually it is measured at the tip of the instrument, which would be the portion of the instrument that most frequently binds in the canal during continuous rotation. Accordingly, a motor-driven tool has been developed which rotates through a defined arc “angle of rotation” in a ‘forward’ direction, which drives the file into the canal much like driving a screw, and a defined (typically lesser) arc of rotation in the “reverse” direction, which in like manner backs the file out of the canal. This reduces opportunities for the file to lock while effectively debriding, cleaning and shaping the root canal for filling. An example of such a tool, is described in U.S. Pat. No. 6,293,795 issued Sep. 25, 2001 to Johnson, which is incorporated herein by reference.
However, during the use of such tools the instrument will repeatedly engage dentine to cut it, and will therefore be repeatedly subjected to torsional stress. This will cause the file material, for example metal or plastic, to undergo structural changes. These changes can be reversible or irreversible, depending on the amount of torque to which the instrument is subjected during canal debridement and on the arc (angle) of rotation to which the instrument is subjected when engaging the tooth structure and binding in the canal. These structural changes will be irreversible if the torque on the instrument is higher than the elastic torque of the instrument (referred to herein as the “elastic limit”), for example when the instrument binds against the canal and the angle of rotation exceeds the elastic angle. In the tool described in U.S. Pat. No. 6,293,795, the torque set on the motor may be higher than the elastic limit of the file; thus, the arcs of rotation in the forward and/or reverse directions are capable of subjecting the file to a torque greater than its elastic limit. Under these conditions, any structural changes in the file material will be irreversible and through repeated use fracture from torsional fatigue, as described above, will eventually occur.
Recently, the use of motor-driven instruments in alternating clockwise and counter-clockwise reciprocation was introduced, to reduce the incidence of fracture from binding (torsional fracture) and fracture from torsional fatigue. An example is described in Yared G. Canal preparation using only one Ni—Ti rotary instrument: preliminary observations. Int Endod J 2008; 41: 339-44 published by the present applicant, which is incorporated herein by reference. Another example is described in U.S. patent publication no. 20120225406 published Sep. 6, 2012 by the present applicant, which is incorporated herein by reference. In this invention, the instrument is rotated alternately in the forward and reverse directions, but the arcs of rotation do not exceed the “elastic angle”, defined as the angle at which the elastic limit of the instrument is reached, which is lower than the angle at fracture. The values of the arcs of rotation, which are lower than the elastic angle, are entered by the operator of the motor. When the instrument reaches the preset arc value (limit) in one direction, the motor will reverse the rotation of the instrument through the arc of rotation in the other direction. Therefore, the instrument will not fracture from binding because the instrument will reverse direction before the instrument reaches an angle at which the instrument can fracture. Fracture from binding (torsional fracture) is therefore substantially eliminated.
Also, if the instrument binds in the canal but is rotated to an angle lower than the elastic angle, torsional fatigue is reduced and consequently fracture from torsional fatigue is reduced. However, these types of endodontic instruments, including new unused instruments, can have surface defects such as corrosion pits and porosities. The repeated cycles of tension and compression to which the instrument is subjected during the canal preparation can initiate cracks in these defects; the cracks will then propagate and eventually cause fracture (Parashos P, Gordon I, Messer H H. Factors influencing defects of rotary nickel- titanium endodontic instruments after clinical use. J Endod 2004; 30: 722-5, which is incorporated herein by reference).
In addition, such motorized tools and files (used in reciprocation to specific arcs of rotation), which are very efficient for non-complex canal situations, are not able to safely address many complex canal anatomies. Despite this fact, dentists are tempted to use motor-driven files with such hand-held motorized devices in reciprocation mode, to enlarge and/or prepare the canal and to address complex canal situations, because of the ease with which they prepare the canal in comparison to manual techniques. This may lead to complications developing. For example, the repeated forward and reverse rotation of the file which does not advance in the canal will subject the file to torsional stress that will accumulate in specific regions, resulting in torsional fatigue and eventually fracture of the file. The present applicant introduced a novel canal preparation technique for root canal treatments and retreatments: (Yared G, 2008; Yared G, 2010: http://endodonticcourses.com/cmsAdmin/uploads/RECIPROC-OL-Article.pdf, which is incorporated herein by reference). In this new concept, preferably a single motor-driven instrument is used in reciprocation mode for the entire canal preparation. Compared to traditional continuous rotation and reciprocation techniques, this novel technique has a shorter learning curve, is faster, and in the majority of the canals requires only one instrument (compared to numerous instruments with the traditional techniques). However, the concern of fracture caused by crack initiation and propagation, and by stress accumulation in complex canal situations, becomes more critical in canal preparation techniques advocating the use of a single motor-driven instrument in a reciprocating mode for the root canal treatment or retreatment procedure (Yared G, 2008; Yared G, 2010: http://endodonticcourses.com/cmsAdmin/uploads/RECIPROC-OL-Article.pdf, which is incorporated herein by reference), because the single instrument, replacing several instruments as used in conventional techniques, is subject to longer periods of torsional stress.
The conventional view is that angles of rotation near the elastic limit of the instrument, at least in the forward direction, are required in order to effectively cut the tooth material. Accordingly, conventional techniques present the risk of instrument fatigue and fracture, with attendant potential complications, as described above. Gambarini teaches “NiTi rotary instruments should be operated only in the superelastic field, a range between the martensite ‘start’ clinical stress values and the martensite ‘end’ clinical stress values, which is a safe and efficient load. Unfortunately, this range is very small and very difficult to determine (16) and this amount of torque might not be adequate for an efficient cutting action, which is strongly influenced by the flute design of the files . . . the elastic and fracture limits of NiTi rotary instruments and their cutting efficiency are obviously dependent on design, dimensions and taper. This means that the right torque values for each individual instrument must be suggested by the manufacturers in order to obtain optimum cutting performance while minimising risks of failure. Unfortunately, it is not an easy task to find such a good balance. As previously mentioned, in some cases predetermined values might be too low to ensure efficient cutting action of the rotary instruments.” Gambarini also states “Theoretically, an instrument used with high torque is very active and negotiation of root canals is easier, even if the incidence of instrument locking and consequent separation would tend to increase. Whereas with low torque, the cutting efficiency would be reduced and instrument progression in the canal would be more difficult. In such cases, if clinicians tended to force the instruments apically, they would increase the chances of locking and separation.” (Gambarini, G. Advantages And Disadvantages Of New Torque-Controlled Endodontic Motors And Low-Torque NiTi Rotary Instrumentation, Australian Endodontic Journal 27, No. 3 December, 2001, which is incorporated herein by reference).
Thompson tested ten instruments ate deflection angles of 360°, 270°, 252°, 216°, 180°, 162°, 144°, 126°, 108° and 90°, but did not consider the results for angles of rotation below 144°, stating “Deflection angles of 126°, 108° and 90° bad greater than 250 cycles to fracture, but during the pilot study, these deflection angles did not allow the instrument to cut and advance into the resin block so they were not included in the experimental groups.” (Thompson, Neil M. Development of a novel canal preparation technique using the torsional fatigue profile of the ProTaper™ F2 rotary instrument, Library and Archives Canada (ISBN: 978-0-494-21105-2) 2006, which is incorporated herein by reference). Prior to the present invention, such angles of rotation were considered to be too low to be effective in a root canal treatment or retreatement.